The boilers of coal-fired steam generators in electric utility plants are typically used in conjunction with crushers to reduce raw coal to chunks or particles of less than a certain size prior to pulverization and introduction to the boiler combustion chamber. To prolong the life of the crusher components, the coal is first passed through a grid made up of parallel bars disposed in the vertical path of a coal chute and spaced apart to pass smaller chunks such that they may be diverted around, rather than through, the crusher. In a given coal stream, this segregation may result in between 40% and 80% of the coal bypassing the crusher.
The grid in the aforementioned apparatus is typically made up of steel bars welded into a grid and disposed in a vertical chute at an angle such that smaller chunks of material pass through while larger chunks of material are caught by the grid and directed into the crusher. In such an operation, some coal chunks are inevitably of such size as to collect on and between the grid bars. Ultimately, such collected materials jam or plug the grid in varying degrees and degrade the efficiency of the segregating operation. To ensure proper operation, these collected materials must be removed from the grid.
For this purpose it is known to use a clearing comb made up of rigid steel fingers normally located in a home position below and behind the grid where it is essentially out of the path of travel of the material which passes through the grid. The comb is periodically actuated by an air cylinder or the like to cause the fingers to lift as a unit and to simultaneously pass upwardly through and between the grid bars to lift and clear the jammed materials from the grid. The air cylinder is then deactivated and the comb drops back to the home position.
While the comb fingers pass between the grid bars, they effectively block the grid and divert all material into the crusher.